As I said, the show is trying.
The Heather episodes were more about Astrid than Hiccup.
Fishlegs gets some mileage out of sharing scenes with Astrid and Hiccup.
Snotlout might be the resident butt monkey, but he gets the most screentime out of it, and the last episode gave some depth to his character. Besides that, his relationship with Hookfang is the most developed of the bunch.
The twins do need more love, though.
And the Twin's Zippleback (whose name I keep forgetting), along with Stormfly could use more characterization. Meatlug also, but to a lesser extent.
All things considering, the show has a better handle on stuff than people give it credit.